This invention relates to a vortex shedding device for use in measuring the rate at which air flows into the air intake of an internal combustion engine. The subject matter of this commonly assigned patent application is related to that of similarly entitled and concurrently filed U.S. application Ser. No. 572,605 in the name of Carlos M. Barrera.
Of the prior art references known by the inventor, U.S. Pat. Nos. 3,722,275 to Rodely et al. and 3,818,877 to Barrera et al. are believed to be the most pertinent to the present invention. To the extent necessary for an understanding of the present invention, the disclosure of U.S. Pat. No. 3,818,877 is incorporated herein by reference. Other patents of interest include U.S. Pat. Nos. 3,756,078 to Yamasaki et al.; 3,863,500 to Yamasaki et al.; and 3,854,334 to Herzl.
Over the years, many attempts have been made to measure continuously the amount of air entering a spark ignition internal combustion engine. This information concerning air intake of the engine can be used to control the amount of fuel metered to the engine or for other engine control purposes. In automotive applications, the engine is operated under varying conditions of engine speed and load and the air intake varies accordingly. Also, numerous transient conditions occur during engine operation. This necessitates, if the air intake flow to the engine is to be measured and used as a control parameter, a flow measuring device that will provide a continuous indication of the instantaneous or substantially instantaneous flow rate into the engine. A satisfactory air flow measuring device must be able to respond over a wide range of air intake flow rates in a substantially linear manner and must have rapid response time and provide a substantially instantaneous indication of such flow rate to permit this to be used as a control parameter in transient conditions of engine operation. Moreover, the metering device must not produce a large pressure drop in the engine air intake system.
In a vortex shedding device for use in measuring air flow rate, typically an elongated element is positioned in the path of the air flow to partially obstruct this flow. Over a range of Reynolds numbers, vortices are generated in the air by such element at a frequency or rate which is proportional to the air velocity. If the number of generated vortices are counted over a known period of time, the air velocity may be easily determined, and, with the element positioned in a conduit of fixed cross-sectional area in the region of the element, an indication of volumetric flow rate is obtained.
In order to apply a vortex shedding device to the measurement of the air intake into an internal combustion engine, it is necessary to obtain an accurate indication of flow rate on a substantially instantaneous basis. Therefore, it is necessary to count the generated vortices for only a very short period of time to provide such substantially instantaneous indication of flow rate. In prior art vortex shedding devices, the frequency of vortex generation has tended to vary over short time intervals even though this frequency, if averaged over a long period such as a few seconds, may be substantially constant. Moreover, the frequency of vortex generation as a function of the air velocity has been quite nonlinear in prior art devices. The present invention overcomes these problems of the prior art.